1. Field of the Invention
The invention relates generally to downhole tools. More particularly, the invention relates to jars for applying an axial impact force to a downhole assembly.
2. Background of the Technology
In oil and gas well operations, it is frequently necessary to apply an axial blow to a tool or tool string that is positioned downhole. For example, application of axial force to a downhole string may be desirable to dislodge drilling or production equipment that is stuck in a wellbore. Another circumstance involves the retrieval of a tool or string downhole that has been separated from its pipe or tubing string. The separation between the pipe or tubing and the stranded tool or “fish” may be the result of structural failure or a deliberate disconnection initiated from the surface.
Jars have been used in petroleum well operations for several decades to enable operators to deliver axial impacts to stuck or stranded tools and strings. “Drilling jars” are frequently employed when either drilling or production equipment gets stuck in the well bore. The drilling jar is normally placed in the pipe string in the region of the stuck object and allows an operator at the surface to deliver a series of impact blows to the drill string via manipulation of the drill string. These impact blows are intended to dislodge the stuck object, thereby enabling continued downhole operations. “Fishing jars” are inserted into the well bore to retrieve a stranded tool or fish. Fishing jars are provided with a mechanism that is designed to firmly grasp the fish so that the fishing jar and the fish may be lifted together from the well. Many fishing jars are also provided with the capability to deliver axial blows to the fish to facilitate retrieval.
Conventional jars typically include an inner mandrel disposed in an outer housing. The mandrel is permitted to move axially relative to the housing and has a hammer formed thereon, while the housing includes an anvil positioned adjacent to the mandrel hammer. By impacting the anvil with the hammer at a relatively high velocity, a substantial jarring force is imparted to the stuck drill string. If the jarring force is sufficient, the stuck string will be dislodged and freed.
There are four basic types of jars: purely hydraulic jars, purely mechanical jars, bumper jars, and mechanical-hydraulic jars. Bumper jars are primarily used to provide a downward jarring force. The bumper jar usually contains a splined joint with sufficient axial travel to allow a pipe to be lifted and dropped, causing the impact surfaces inside the bumper jar to come together to deliver a downward jarring force to the string.
Mechanical, hydraulic, and mechanical-hydraulic jars differ from the bumper jar in that each contains a triggering mechanism which prevents impacting each other until a sufficient axial strain, either tensile or compressive, has been applied to the jar. To provide an upward jarring force, the drill pipe is stretched by an axial tensile load applied at the surface. This tensile force is resisted by the triggering mechanism of the jar long enough to allow the string to stretch and store potential energy. When the jar triggers, this stored energy is converted to kinetic energy causing the impact surfaces of the jar to move together at a relatively high velocity. To provide a downward jarring force, the pipe weight is slacked off at the surface and, and in some cases, additional compressive force is applied, to place the string in compression. This compressive force is resisted by the triggering mechanism of the jar to allow the string to compress and store potential energy. When the jar triggers, the potential energy is converted to kinetic energy causing the impact surfaces of the jar to come together at a relatively high velocity.
The triggering mechanism in most mechanical jars consists of a friction sleeve coupled to the mandrel which prevents movement of the mandrel relative to the housing until the load applied to the mandrel exceeds a preselected amount, often referred to as the “triggering load.” The triggering mechanism in most hydraulic jars consists of one or more pistons which pressurize fluid in a chamber in response to movement by the mandrel relative to the housing. The compressed fluid resists movement of the mandrel. The pressurized fluid is ordinarily allowed to bleed off at a preselected rate. As the fluid bleeds off, the mandrel slowly translates relative to the housing, eventually reaching a point in the jar where the chamber seal is opened, and the compressed fluid is allowed to rush past the piston, thereby allowing the mandrel to move rapidly.
Mechanical-hydraulic jars ordinarily combine some features of both purely mechanical and purely hydraulic jars. For example, one design utilizes both a slowly metered fluid and a mechanical spring element to resist relative axial movement of the mandrel and the housing. Another design utilizes a combination of a slowly metered fluid and a mechanical brake to retard the relative movement between the mandrel and the housing. In this design, drilling mud is used as the hydraulic medium. Therefore, the string must be pressurized before the jar will operate. This pressurization step will ordinarily require a work stoppage and the insertion of a ball into the work string to act as a sealing device. After the jar is triggered, the ball must be retrieved before normal operations can continue.
In many wireline retrieval operations, particularly tight hole operations, it is often desirable to applying a tensile load on the wireline in an attempt to free the stuck downhole object without firing the jar. For example, the operator may slowly increase tension on the wireline, and then hold the tension for an extended period of time to try and dislodge the downhole assembly without the need for triggering the jar. In some cases, the operator may choose apply an overload tension in excess of the triggering load of the jar to try and dislodge the downhole assembly, but not want to fire the jar. However, with most conventional jars, application of a tensile load over a long period of time and application of an overload tension are likely to cause the jar to inadvertently fire or be very near the point of firing.
Accordingly, there remains a need in the art for downhole jars and associated devices that allow the jar triggering load to be exceeded for a finite period of time without causing the jar to fire. Such jars and associated devices would be particularly well-received if they provided the operator the option of reducing the line tension shortly after the overpull to avoid jarring, or maintaining the overpull to fire the jar.